This invention relates to a method and apparatus for controlling the position of a rotary head which is mounted on a deflection device and which scans successive tracks on a record medium and, more particularly, to such a method and apparatus whereby the head is controlled to scan traces which are made to conform either to a predetermined reference during, for example, a signal recording operation, or to conform to previously recorded tracks during, for example, an edit operation.
In video signal recording/reproducing apparatus, video signals are recorded in successive, skewed tracks. Typically, the record medium upon which the video signals are recorded is a magnetic tape; and the recording/reproducing apparatus are video tape recorders (VTRs) of the so-called helical scanning type. Preferably, in order to increase the amount of information which can be recorded on a magnetic tape of fixed length and, thus, to increase the recording time of a video tape, it is advantageous to record the video signals with higher recording densities.
One proposal for increasing the recording density on a magnetic tape is to record the video signals with reduced wavelength. Magnetic materials have been developed, recording and/or reproducing transducers have been manufactured, and new modulation systems have been designed so as to implement short wavelength recording of video signals. However, when recording signals with a short wavelength, a limit on the reduction of the wavelength soon is reached below which information losses rapidly increase. For example, below this limit, the signal-to-noise ratio rapidly decays in a function related to the second or third power.
An alternative technique for improving the recording density at which video signals may be recorded contemplates the recording of such signals in relatively narrow tracks having small or no guard bands therebetween. Such narrow tracks, and particularly tracks which are adjacent each other, require good control over the transducers so that the traces scanned thereby coincide with the desired tracks. An advantage in using narrow tracks is that, even as the width of the tracks is decreased, the signal-to-noise ratio decays at a much slower rate than that when shorter wavelengths are used. Typically, the S/N ratio decays relative to the width of the tracks in a function that approximates the square root.
Recently, automatic tracking systems have been proposed for controlling the scanning traces of a transducer which, for example, reproduces video signals from relatively narrow tracks. One such automatic tracking control system is described in U.S. Pat. No. 4,172,265. With this patented system, the transducer is controlled within a few microns of the track being scanned thereby, even if the track itself exhibits a distortion, or deformation, on the order of about 100 microns. That is, if the track is not perfectly linear but, in fact, bends or twists so as to be deformed from a perfectly straight track, the patented scanning control system nevertheless controls the position of the transducer so as to scan such a deformed track accurately.
In addition to the aforementioned system, other proposals have been made for controlling the scanning trace of the transducer during signal reproduction. Examples of such other proposals are found in U.S. Pat. Nos. 4,099,211 and 4,106,065.
However, in these and other tracking control arrangements, little, if any, control is effected over the transducer during recording operations. That is, although the transducer is positioned properly so as to scan previously recorded tracks, when the transducer operates to record information, there is little position control thereover. While this generally does not present a problem during normal recording, it should be avoided when the VTR operates in an edit mode, whereby new video signals are "inserted" between two segments of previously recorded signals or, alternatively, new video signals are "assembled" immediately following a segment of previously recorded signals. In particular, those signals which are recorded immediately following the edit-point, that is, those signals which are recorded immediately following the change-over of the VTR from its reproducing to its recording mode, might be recorded in tracks which are so dissimilar from the previously recorded tracks as to make continuous reproduction across the edit point difficult. For example, the previously recorded signals may be recorded in tracks which appear distorted, whereas the new, or edited, signals are recorded in tracks whose configuration appears relatively linear. This may occur if the video tape "stretches" or is otherwise distorted from the time that the previously recorded signals were recorded to the time that the new signals are recorded. Likewise, this may occur if the previously recorded signals were recorded by one VTR and the new signals are recorded by another. Also, because of changes in the operating conditions of the VTR, deformation of the tape, or other factors, the configuration of the previously recorded tracks may differ significantly from the configuration of the newly recorded tracks such that the transducer undergoes a discontinuity across the edit point.
In the foregoing examples, when the edited video tape is reproduced, the usual tracking control arrangement may be effective to conform the traces of the transducer to the configurations of the tracks on either side of the edit point; but a perceptible tracking error will be present at the edit point. This is because the transducer must scan the previously recorded tracks of one configuration and then, abruptly, must scan the newly-recorded tracks of another configuration. This perceptible tracking error is even more pronounced when tracking control systems of the so-called correlating type are used. That is, if the configuration of the newly-recorded tracks is predicted on the basis of the configuration of the previously recorded tracks, the tracking error at the edit point will be more pronounced.